Pythium. It’s not quite a fungus. It resembles algae. There are several hundred species, which are genetically diverse. Some are plant pathogens, often with a wide host range. Those that cause plant diseases are responsible for an array of diseases, which can infect many crops. Pythium spp. can causes symptoms similar to other pathogens, including damping off, root and stem rot and leaf blight.

Mode of action

Pythium spp. causes infection via a plant’s root system. It can cause infection in several ways. Some species have an asexual reproduction phase, with motile zoospores that swim through water to enter root tips. Others infect with oospores, the result of sexual reproduction. The life cycle of other Pythium spp. includes both types of reproduction and both routes of infection.

The exact route of infection “kind of depends on what Pythium species you have,” Dr. Len Tesoriero, University of Australia senior plant pathologist, said in a webinar. “Oospores are very resilient. They can survive in the soil for up to 15 years or more,” while zoospore “are chemically and electrically attracted to things like roots.”

The plant’s rhizosphere is an area that contains “lots of nutrients,” which serve to attract zoospores to the region, Tesoriero said. The zoospores propel themselves through water toward these attractants. For zoospores, “water is the key,” so hydroponically grown crops are at risk, too.

Zoospores can swim for over 20 hours. Although they normally die rapidly without water, in some cases they can form a hard shell, enabling them to survive for up to a week. Zoospores, attracted to root tips, can infect a plant very quickly. In turn, the damaged, leaky roots emit substances, such as amino acids, which further attract other zoospores.

Oospores are able to survive in plant debris or soil for over a decade. Pythium spp. are common in soils and in water sources and can be found in sediment and in decayed plant material.

The pathogens can infect all stages of a plant’s development. No matter the stage of the plant, when Pythium infection occurs, the roots will be severely impacted. Plants with stronger root systems, or which rapidly produce new roots, are more able to survive Pythium infection.

With a wide range of plant hosts, plus the ability to act in conjunction with other plant pathogens, such as Fusarium, Pythium spp. can cause widespread plant disease. Alarmingly, there is evidence that Pythium spp. also act synergistically with herbicides, such as glyphosate, and soil residues of the herbicide can enhance Pythium spp. pathogenicity.

Pythium spp. are “synergists with other plant pathogens and pests,” Tesoriero said.

Symptoms of infection

If seeds are infected, preemergent rot occurs. Seedlings and post-transplant young plants exhibit damping off and root rot. Fungus gnat larvae and shorefly adults are thought to transport Pythium, particularly in greenhouse settings, soilless mediums or soils high in organic manner. Some insects ingest Pythium spores and then excrete them.

Damping off is “probably one of the most common things,” making Pythium a concern for greenhouse growers, Tesoriero said.

Infections can occur rapidly, as the organism population can increase 1,000-fold in under an hour when conditions are conducive. But other diseases can cause damping off and root rot of seedlings and transplants, too, so the symptoms don’t necessarily point to Pythium spp. infection.

Pythium spp. can cause many other concerns beyond the seedling stage, and the symptoms may not always be as obvious. The symptoms of disease vary, with Pythium spp. causing fruit rot, stem rot, leaf blight, replant diseases, wilt and root diseases. Because plants can be infected at all life stages, the severity of the disease can depend on the plant’s maturity.

Some Pythium spp. can move up into plant tissues, causing diseases such as the post-transplant disease of lettuce leaf blight. Initial symptoms of such Pythium spp. infection may be subtle, including slight discoloration or small size of plants.

“Quite often you wouldn’t even know that Pythium is infecting your plant,” Tesoriero said.

Many Pythium species can attack many plant hosts, while others are more host-specific. Some examples of Pythium diseases common in vegetable crops:

  • Lettuce: Post-transplant, leaf blight and wilts can occur due to Pythium uncinulatum infection. Pythium move from the roots into the plant and cause plant collapse.
  • Carrots: Cavity spot is caused by Pythium sulcatum, which creates lesions on the mature taproots.
  • Spinach: Pythium often acts in conjunction with Fusarium, phytophthora and rhizoctonia in a complex system, causing wilt disease of spinach.
  • Cucumber: Pythium spp. can cause cottony leak in fruits.


There is little genetic resistance to Pythium spp. in any vegetable crop. Pythium spp. can also affect grain and alfalfa, as well as non-crop plants, and some can cause livestock disease in wet pastures.

Pythium spp. thrive in wet soils or in water. The degree of infection depends on the density of zoospores and oospores in the medium, as well as the temperature, nutrient levels and aeration of the soil. Some Pythium species thrive under cool conditions, while others proliferate rapidly in warm conditions. The intensity of light, the presence of other organisms or pests and any physical damage to plants also impact the severity of disease.

“Pythium tends to tolerate low aeration conditions around the root zone,” Tesoriero said. “It out-competes other organisms,” such as beneficial microbes, as soil moisture levels increase.

There are chemical controls to combat Pythium spp. But rapid recolonization and resistance are common. Because Pythium travels through water, it is easily reintroduced into a cropping system. Biological control agents and fungicides can be used preventively in potting mix and trays.

Biological control agents can produce antibiotics, which are effective against Pythium spp. Other biological controls, such as Trichoderma spp., will parasitize the pathogen. Biodetergents can dissolve away the flagella or the zoospore. Competitors on the root system, including other Pythium species, will attack plant pathogenic Pythium spp. by exuding enzymes that break down cell walls. Many bacteria and fungi can also induce systemic plant resistance.

Other nonchemical means of control include the chemical properties of the soil. Enhanced calcium levels in plants have been shown to be beneficial to decrease Pythium spp. infections.

“Soils that have got a higher exchangeable cation level, then you’ll get less Pythium disease,” Tesoriero said.

Cultural controls include sowing seeds when temperatures are not extreme and when using fresh seed. Disinfecting water sources, improving irrigation scheduling and enhancing drainage can be effective in reducing Pythium spp. density.

Determining if Pythium spp. is causing your symptoms, alone or in conjunction with other organisms, can be difficult. According to information from Penn State University’s Department of Plant Pathology: “One way to determine quickly whether the organism you are examining is a Pythium or determine that the pathogen affecting a plant is Pythium is to employ a kit available from Neogen (ALERT-LF). This kit is easy to use and gives a result in about 20 minutes. A positive test tells you that the organism present is probably in the genus Pythium. There are no kits for identifying the species present.”

To view Tesoriero’s webinar, go to the website.